Surface and Coating Analysis of Press‐Hardened Hot‐Dip Galvanized Steel Sheet

The use of continuous galvanized steel sheet as feedstock material for press hardening leads to components of very high strength levels with classical cathodic corrosion protection. The present work provides an insight into this technology with special focus on surface oxidation and intermetallic phase formation during the austenisation process. For that reason hot dipped galvanized 22MnB5 steel sheets, with a blank thickness of 1.5 mm and an average coating weight of 70 g m^?2, were annealed in a temperature range of 400–900°C in steps of 50°C without soaking before quenching in water. Surface and cross‐cuts were analyzed by SEM, EDX, and XRD to illustrate the phase formation and the surface changes during thermal treatment. Corrosion behavior was also studied based on electrochemical investigations and an accelerated, cyclic, automotive corrosion tests called VDA, which is a mixture of salt spray test and alternating climate test. It was found that austenisation of galvanized steel sheet causes a significant change of the coating. The generated coating still offers cathodic protection for the steel substrate and has higher corrosion resistance than standard galvanized steel sheet. Surface oxidation also occurs during the press hardening process leading to a surface covered with successively arranged layers of Al₂O₃ and ZnO, containing also further oxidized alloying elements.     

Liquid-Metal-Induced Embrittlement of Zn-Coated Hot Stamping Steel

Liquid-metal-induced embrittlement (LMIE) of galvanized hot stamping steel occurs due to the simultaneous application of stress and the presence of a liquid Zn surface layer during the hot stamping process. The mechanism specific to the liquid metal induced embrittlement occurring during hot stamping was investigated in detail. It was found that when a tensile stress was applied, liquid Zn could penetrate along grain boundaries in the steel matrix at temperatures above the Liquid?+???-Fe (Zn) ?? ??₁ peritectic transformation temperature of 1055?K (782?°C). The results show that an increase of the annealing time prior to hot stamping is an effective way to prevent LMIE by the elimination of the liquid phase.     

先进镀锌高强钢中液态金属脆化的研究进展及挑战

先进高强钢广泛用于汽车车身结构,能够在保证车辆安全性的同时使整体车身减重,满足节能减排的要求,在汽车轻量化方面已凸显出巨大的应用潜力.通常汽车用高强钢板需要进行表面镀锌处理以提高车身结构的抗腐蚀能力.但镀锌高强钢板在点焊过程中容易产生液态金属脆化,降低焊接接头的承载性能.镀锌热成形高强钢在热成形过程中也会产生液态金属脆...     

中锰钢温成形技术探讨

使用Gleeble-3800热模拟试验机研究了中锰钢(温成形钢)和22MnB5钢(热成形钢)的微观组织、力学性能和高温拉伸性能.结果表明:与22MnB5钢相比,在获得1 500 MPa级的抗拉强度时,中锰钢的加热温度可从950℃降低到800℃,钢的组织明显细化并且没有发生表面脱碳,断后伸长率从7.5％提高到10％.高温拉伸试验结果表明:中锰钢比22MnB5钢具有更高的延伸率和硬化指数,可以减小成形过程中局部减薄过高导致的样件破裂.     

Microstructural evolution of Al-Si coating and its influence on high temperature tribological behavior of ultra-high strength steel against H13 steel

Al-Si coated ultra-high strength steel(UHSS)has been commonly applied in hot stamping process.The influence of austenitizing temperature on microstructure of Al-Si coating of UHSS during hot stamping process and its tribological behavior against H13 steel under elevated temperature were simulatively investigated.The austenitizing temperature of Al-Si coated UHSS and its microstructual evolution were confirmed and analyzed by differential scanning calorimetry and scanning electron microscopy.A novel approach to tribological testing by replicating hot stamping process temperature history was presented.Results show that the hard and stable phases Fe_2Al_5+FeAl_2 formed on Al-Si coating surface after exposure to 930°C for 5 min,which was found to be correlated to the tribological behavior of coating.The friction coefficient of coated steel was more stable and higher than that of uncoated one.The main wear mechanism of Al-Si coated UHSS was adhesion wear,while abrasive wear was dominant for the uncoated UHSS.     

Martensitic Transformation Behavior of B‐Bearing Steel During Isothermal Deformation

The purpose of the present research is to study the martensitic transformation in 22MnB5 steel under thermomechanical conditions by means of dilatation data. To reach this aim, the effects of deformation temperature and strain rate on the martensitic dilatation as well as martensite start temperature (M_s) were investigated. Thermomechanical treatments were performed in a deformation dilatometer including the isothermal deformation of samples in the temperature range of 550–900°C up to the final strain of 0.5 in three strain rates of 0.1, 1, and 10 s^?1. Finally, deformation temperatures were divided into two regimes of lower and higher than 800°C. In the former, strain‐induced phase transformations, while in the latter, occurrence of dynamic recovery against mechanical stabilization of austenite influenced martensitic transformation.     

Corrosion Resistance of Different Metallic Coatings on Press‐Hardened Steels for Automotive

The corrosion resistance of laboratory press‐hardened components in aluminized, galvanized or galvannealed boron steels was evaluated through VDA 621‐415 cyclic test for the automotive industry. 22MnB5 uncoated steel for hot stamping and standard galvanized steel for cold forming were also included as references. Corrosion resistance after painting (cosmetic corrosion) was quantified by measuring the delamination of electro‐deposited paint from scribed panels. The rusting on their edges was used for determining the cut‐edge corrosion resistance. The corrosion resistance on unpainted deformed panels (perforating corrosion) was quantified by mass losses and pit depth measurements. Zinc‐coated boron steels were found to be more resistant to cosmetic corrosion than the other materials, and slightly more resistant to cut‐edge corrosion than the aluminized one. Red rust apparition could not be avoided due to the high iron content in all these hot‐stamped coatings. The three coated boron steels showed similar performances in terms of resistance to perforation. Aluminized boron steel presents the advantage of being less sensitive to hot‐stamping process deviation. Its robustness has been proved for many years on cars.     

热镀锌液中Al含量及工艺对TRIP钢表面质量的影响

通过对TRIP590钢进行不同Al含量锌液热镀锌对比试验,研究了锌液中不同Al含量及热镀锌工艺对TRIP钢镀层性能的影响。试验结果表明,镀锌液中加入5%Al,均热退火温度800℃,可明显提高TRIP钢的镀层质量,改善热镀TRIP钢的抗腐蚀性能。     

Influence of Isothermal Deformation Conditions on The Mechanical Properties of 22MnB5 HPF Steel

In the present study, the aluminized 22MnB5 Hot Press Forming (HPF) steel was deformed isothermally in the temperature range of 600–800 °C. The specimens were deformed with 0%, 10% and 30% engineering strain at a strain rate of 0.5/s. Mechanical stabilization of austenite during phase transformation was observed. The influence of strain rate was also studied. It was found that the martensite transformation was affected by the combined effect of mechanical stabilization and dynamic recovery. The influence of high temperature deformation on the final microstructure, microhardness and mechanical properties were studied. Deformation applied below 750 °C resulted in a significant decrease of the strength and the hardness. The ferrite fraction and morphology had a clear effect on the room temperature tensile fracture morphology.     

Influence of Gas Atmosphere Dew Point on the Galvannealing of CMnSi TRIP Steel

The Fe-Zn reaction occurring during the galvannealing of a Si-bearing transformation-induced plasticity (TRIP) steel was investigated by field-emission electron probe microanalysis and field-emission transmission electron microscopy. The galvannealing was simulated after hot dipping in a Zn bath containing 0.13 mass pct Al at 733 K (460 °C). The galvannealing temperature was in the range of 813 K to 843 K (540 °C to 570 °C). The kinetics and mechanism of the galvannealing reaction were strongly influenced by the gas atmosphere dew point (DP). After the galvannealing of a panel annealed in a N₂+10 pct H₂ gas atmosphere with low DPs [213 K and 243 K (?60 °C and ?30 °C)], the coating layer consisted of δ (FeZn₁₀) and η (Zn) phase crystals. The Mn-Si compound oxides formed during intercritical annealing were present mostly at the steel/coating interface after the galvannealing. Galvannealing of a panel annealed in higher DP [263 K and 273 K, and 278 K (?10 °C, 0 °C, and +5 °C)] gas atmospheres resulted in a coating layer consisting of δ and Г (Fe₃Zn₁₀) phase crystals, and a thin layer of Г ₁ (Fe₁₁Zn₄₀) phase crystals at the steel/coating interface. The Mn-Si oxides were distributed homogeneously throughout the galvannealed (GA) coating layer. When the surface oxide layer thickness on panels annealed in a high DP gas atmosphere was reduced, the Fe content at the GA coating surface increased. Annealing in a higher DP gas atmosphere improved the coating quality of the GA panels because a thinner layer of oxides was formed. A high DP atmosphere can therefore significantly contribute to the suppression of Zn-alloy coating defects on CMnSi TRIP steel processed in hot dip galvanizing lines.     

Elevated temperature oxidation of laser surface engineered composite boride coating on steel

The effects of long duration exposure of laser surface engineered composite boride coating on plain carbon steel in air at high temperatures were investigated in this study. Exposures at 600 °C, 800 °C, and 1000 °C for 10, 30, and 50 hours of composite-TiB₂ coated samples were conducted to study oxide scale growth and morphology. Kinetics of oxidation of the coating during elevated temperature exposures were separately studied using the thermogravimetric analysis (TGA) technique. The oxidation rate for all samples was parabolic in nature and the oxidation kinetic rate constant, K, increased with increasing temperature of exposure. Activation energy, Q for composite TiB₂ coating was found to be 205 kJ/mol. A thick (>35 μm) oxide layer formed for all duration of exposure at temperatures ≥800 °C. In case of 1000 °C exposure, a very thick (>150 μm) oxide layer was formed, which was separated from the substrate. X-ray diffractometry analysis revealed the complex nonstoichiometric nature of the oxides of type Ti _a O _b , Fe _m O _n , and Fe _x Ti _y O _z . Profilometric measurements indicated an increase in the surface roughness of the oxide layer with an increase in temperature of exposure. These physical observations indicated that the nature and morphology of the oxides formed at various temperatures and duration of exposure are complex.     

Analysis of Microstructure and Mechanical Properties of Different Hot Stamped B‐bearing Steels

Hot stamping is a technique to produce ultra high strength automobile components. The common material used in hot stamping process is coated and/or uncoated 22MnB5 boron alloyed steel. Ferritic‐pearlitic microstructure in as‐delivered sheets is transformed to fully lath martensitic after hot stamping. In the present research, hot stamping under water or nitrogen cooling media was investigated using different boron alloyed steel grades. Microstructural analyses, linear and surface hardness profiling as well as tensile tests of hot stamped samples were performed. Various microstructures of fully bainitic and/or fully martensitic were produced. The resulting microstructures provided yield strengths of 650–1370 MPa and tensile strengths of 850–2000 MPa. There is an optimum carbon equivalent content for which the highest formability index value, UTS × A₂₅, is achieved. Using a nitrogen cooled punch resulted in higher yield strength without significant changes in ultimate tensile strength. It is concluded that a wide range of B‐bearing steels having an extended carbon equivalent range with an acceptable formability index value can be used by increasing the cooling rate in the die assembly.     

Effect of NiCl_2-Based Fluxes on Interfacial Layer Formation of Hot Dip Galvanized Steels

While the thick growth of intermediate phase layers generally benefits the corrosion resistance of galvanized steels,it is unfavorable from the standpoints of mechanical integrity and economics.Thus,the influence of nickel chloride-based fluxes and the typical zinc-ammonium chloride flux on galvanized coating thickness as well as coating morphology and composition is examined.The investigation of pretreated hot dip galvanized steel specimens for various durations has verified that nickel chloride fluxes inf...     

Structure formation in high-strength nitrogen-bearing steel on hot deformation

The resistance to deformation of nitrogen-bearing Cr–Ni–Mn steel at 800–1200°C is investigated by means of the Gleeble 3800 system. By analysis of the deformation diagrams—in particular, determination of the threshold strain for dynamic recrystallization—the temperature and strain corresponding to the onset of dynamic recrystallization are established as a function of the strain rate, and optimal temperatures for hot stamping, forging, and rolling are recommended for industrial conditions. With true strain e = 0.9, the dynamic recrystallization in the steel at strain rates of 10^–2–2 s^–1 occurs at temperatures no lower than 900°C. Metallographic data confirm the experimental results and show that the structure formation in the steel on isothermal deformation at different rates is different above 900°C. With increase in temperature and decrease in strain rate, relaxation processes are more developed. At a strain rate of 0.01 s^–1 (stamping on a press), dynamic recrystallization begins at around e = 0.1 (relative reduction around 10%) in the range 1100–1200°C. Strain of around 20 and 30%, respectively, is required with decrease in temperature to 1000 and 900°C. With increase in strain rate to 0.1 s^–1 (forging), dynamic recrystallization begins with around 20% strain above 1100°C, 28% at 1000, and 35% at 900°C. At a strain rate of 1–2 s^–1 (rolling), dynamic recrystallization begins at around 30% strain in the range 1000–1100°C. In that case, the threshold strain is 36% at both 900 and 1200°C.     

The Coexistence of Two Different Pearlites,Lamellae of (Ferrite + M<Subscript>3</Subscript>C), and Lamellae of (Ferrite + M<Subscript>23</Subscript>C<Subscript>6</Subscript>) in a Mn-Al Steel

Two different pearlites after two separate eutectoid reactions were observed in an Fe-19.8 Mn-1.64 Al-1.03 C (wt pct) steel. The steel specimens were processed under solution heat treatment at 1373 K (1100 °C) and received isothermal holding at temperatures from 1073 K to 773 K (800 °C to 500 °C). The constituent phase of the steel is single austenite at temperatures between 1373 K and 1073 K (1100 °C and 800 °C). At temperatures below 1048 K (775 °C), M₃C and M₂₃C₆ carbides coprecipitate at the austenitic grain boundaries. Two different pearlites appear in the austenite matrix simultaneously at temperatures below 923 K (650 °C). One is lamellae of ferrite and M₃C carbide, and the other is lamellae of ferrite and M₂₃C₆ carbide. These two pearlites are product phases from two separate eutectoid reactions, i.e., austenite → ferrite + cementite and austenite → ferrite + M₂₃C₆. Therefore, the supersaturated austenite has decomposed into two different pearlites, separately.     

温成形中锰钢的组织性能评价

 热成形零件已在汽车安全件上广泛应用,为了进一步提升零件碰撞安全性、提高表面质量、降低成本,基于中锰钢提出了一种降低加热温度的热成形技术,通过将完全奥氏体化的中锰钢在模具中淬火成马氏体组织获得超高强度力学性能,与22MnB5钢热成形相比,在获得1 500 MPa抗拉强度时,中锰钢温成形的加热温度可降低150 ℃以上,断后伸长率提高30%以上,同时提高零件的表面质量。综述并评价了中锰钢经温成形后的微观组织与力学性能以及冷弯性能、成形性能、电阻点焊等工艺性能,并与22MnB5钢热成形进行了系统地比较,体现出温成形中锰钢节能环保、提高碰撞安全性的技术优势。     

Modeling of Microstructure Evolution in 22MnB5 Steel during Hot Stamping简

Automobile manufacturers have been inereasingl~r adopting hot-stamped parts for use in newly designed ve- hicles to improve crash worthiness and fuel efficiency. However, the simulation of hot stamping is rather complex and challenging, and further research still needs to be done on hot stamping hardening mechanism. The microstruc- ture evolution and hardening mechanisms during hot stamping of 22MnB5 steel were thoroughly investigated, using information provided in the literatures as well as experimental results. New models were developed to predict the grain growth during heating and the flow stress of a manganese boron steel （22MnB5） with high hardenability by the Gleeble simulation experimental results. The deformed austenite decomposition during stamping and quenching was emphatically quantified based on the transformation thermodynamic and kinetic theories, and the relationship of mi- crostructure to properties was analyzed. The results showed that the optimal process to obtain homogeneous and small lath martensite is heating at 900--950 ℃ for 5 min and then auenching at 50 ℃/s with a Dressing time about 8 s.     

Galvanizability of Complex Phase High Strength Steel

Complex phase (CP) steels have very high ultimate tensile strengths, resulting from the use of specific alloying elements, which improve the hardenability but cause difficulties when applying a zinc coating by means of continuous hot‐dip galvanizing. The galvanizability of a cold rolled 1000MPa complex phase steel was investigated by monitoring the surface chemistry before dipping and evaluating the quality of the zinc coatings applied by a laboratory hot‐dipping simulator. Two steel compositions with different Cr levels were used. The influence of the most important production parameters, the annealing temperature and the dew point of the annealing atmosphere, was investigated. Both steel compositions were galvanizable, but both the surface appearance and zinc coating adhesion were improved when low Cr contents were used. At a low dew point of ‐30°C, Cr, Mn and Si segregated to the surface and the presence of Mn₂SiO₄ could be demonstrated. At high dew point of +10°C, less oxides were present at the steel surface. There was no effect of the annealing temperature on the coatability.     

Optimization Evaluation Test of Strength and Toughness Parameters for Hot-Stamped High Strength Steels

Use of hot-stamped high strength steels (HSHSS) not only reduces the vehicle weight, but also improves the crash safety, therefore more and more mentioned steels are used to produce automobile parts. However, there are several problems especially the low ductility and toughness, which have restricted the application of HSHSS in automobile body. Suitable process parameters are very crucial to improve strength and toughness. In order to study the effect of austenization temperature, soaking time and start deformation temperature on strength and toughness of boron steel 22MnB5, an L9 (3⁴) orthogonal experiment which was analyzed by means of comprehensive evaluation was carried out based on Kahn tear method to obtain the value of fracture toughness. The results indicate that the excellent formability, high strength and toughness of boron steel 22MnB5 with 1.6 mm in thickness are obtained when the austenization temperature is in the range of 920 — 950 °C, the soaking time is 1 min and the start deformation temperature is in the range of 650 — 700 °C. The optimal parameters were used for typical hot stamping structural parts tests. Properties of samples such as tear strength, unit initiation energy and ratio of strength to toughness (RST) were improved by 10.91%, 20.32% and 22.17%, respectively. Toughness was increased substantially on the basis of a small decrease of strength.     

一种Si-Mn系超高强度钢板的热冲压成形试验研究

 采用弯曲件热冲压成形试验研究了板料加热温度、保温时间及移送时间等工艺参数对一种Si-Mn系超高强度钢板热成形零件的力学性能及微观组织的影响规律。结果表明,通过控制热成形工艺参数,在所设计的模具上可实现Si-Mn系超高强度钢板热成形零件的有效淬火,在合适的工艺参数下,可获得细小均匀的马氏体组织,从而获得抗拉强度1700MPa以上,伸长率10%以上的性能,达到原始板料抗拉强度的3倍左右,并明显高于传统的Mn-B系超高强度钢板。